The present invention lies in the field of semiconductor technology and relates to an integrated circuit having a field effect controllable trench transistor (FET) having two control electrodes.
Conventional field-effect transistors include a first connection electrode (e.g., source electrode) connected to a first connection zone (e.g., source connection zone) of a semiconductor body, and a second connection electrode (e.g., drain electrode) connected to a second connection zone (e.g., drain connection zone) of the semiconductor body. Furthermore, a control electrode (“gate electrode”) insulated from the semiconductor body is provided for controlling a conductive channel between these two electrodes, the control electrode being connected or being able to be connected to a control potential (“gate potential”). In the case of a trench transistor, the control electrode extends in a trench into the semiconductor body. A gate insulation layer serves for insulating the gate electrode from the semiconductor body.
A channel zone (“body zone”) doped complementarily with respect to the drain and source connection zones can be arranged between the two connection zones in order to make the FET normally off. In this case, the gate electrode is arranged adjacent to the body zone in order to produce a conductive channel (“inversion channel”) in the body zone upon application of a control potential having a suitable sign, thereby enabling a current flow in the semiconductor body when voltage is applied between the source electrode and the drain electrode. In the case of a normally on FET, a complementarily doped body zone is not provided, the control electrode in this case serving to pinch off a conductive channel through application of a control potential having a suitable sign in order to interrupt a current flow in the semiconductor body when a supply voltage is applied to drain electrode and source electrode.
For the practical application of field effect transistors, endeavors are made to ensure that they have a high breakdown voltage (dielectric strength) and at the same time a low on resistance. In this case, the breakdown voltage is defined as that supply voltage at which a normally off FET undergoes transition to breakdown with the gate electrode not being driven. With the gate electrode being driven, the on resistance can be determined computationally as the quotient of the applied supply voltage and the source-drain current that occurs at this voltage.
Furthermore, for practical application endeavors are made to ensure that the field effect transistors have a minimum parasitic capacitance between the gate electrode and the drain electrode since the switching losses, particularly at high switching frequencies, are increased thereby in a disadvantageous manner. Fast switching edges advantageously enable for example, the operation of a DC/DC converter at higher switching frequencies, so that the efficiency can be increased and the system outlay can be reduced overall. In the case of trench transistors, the parasitic capacitance between the gate electrode and the drain electrode in particular also increases with an increasing overlap region of gate electrodes and drain zone.
As a measure for increasing the breakdown voltage, it is known for example, to provide a thicker gate insulation layer between the gate electrode and semiconductor body, but this increases the on resistance in a disadvantageous manner. Furthermore, for this purpose it is known to provide an epitaxial zone (drift zone) doped more weakly than the drain connection zone between the body zone and the drain connection zone, but this increases the on resistance in a disadvantageous manner. The drain connection zone and the more weakly doped drift zone can be combined under the term “drain zone”.
In order to realize a maximum breakdown voltage in conjunction with a low on resistance and a low parasitic capacitance between gate electrode and drain electrode and hence low switching losses, in trench transistors the use of a further control electrode (“field electrode”) arranged adjacent to the gate electrode in the trench has been proposed, the further control electrode being insulated from the semiconductor body and being able to be connected to a field electrode potential. In this case, the field electrode preferably extends through the majority of the drift zone. A field electrode insulation layer serves for insulating the field electrode from the semiconductor body, and is advantageously thicker than the gate insulation layer.
In such trench transistors having two control electrodes, when the supply voltage is present the field electrode reduces a field strength acting at the gate insulation layer of the gate electrode. In this way, relative to comparable field effect transistors without such a field electrode given the same dielectric strength, the doping of the drift zone can be increased, whereby the on resistance and the parasitic capacitance between the gate electrode and the drain electrode can advantageously be reduced. In this case, in particular the potential present at the source electrode is applied to the field electrode.
In the case of such trench transistors having a gate electrode and an additional field electrode connected to source potential, it is possible, in principle, to set the parasitic capacitance between the gate electrode and the drain electrode (that is to say the gate-drain charge) by using the position of the lower edge of the gate electrode in relation to the pn junction, that is to say the junction from the body zone into the drain zone (drift zone). In this case, it is particularly advantageous if, as viewed in the direction of the trench depth, the lower edge of the gate electrode is situated as precisely as possible at the level of the pn junction, because this makes it possible to achieve a minimum parasitic capacitance between the gate electrode and the drain electrode.
Furthermore, U.S. Pat. No. 4,941,026 discloses, instead of two separate control electrodes, the formation of a gate electrode that extends right into the drift zone and is surrounded by a thin-thick oxide. One advantage of such a gate electrode is primarily a relatively low on resistance on account of the formation of an accumulation channel, the on resistance being reduced by approximately 10 to 30% relative to transistors having a gate electrode that does not extend right into the drift zone. One disadvantage, however, is the accompanying higher parasitic capacitance between the gate electrode and the drain electrode, the parasitic capacitance being increased by approximately 50 to 300% relative to transistors having a gate electrode that does not extend right into the drift zone. Moreover, preceding from a process for forming a transistor including separate gate and field electrodes, a high development outlay is necessary since, by way of example, after the etching of the field oxide at the upper trench sidewalls and during the growth of the gate oxide, an oxide layer is formed on the field electrode and insulates the latter from the gate potential. Subsequent processes for selectively removing the oxide layer on the field electrode contaminate or thin the gate oxide and lead to reliability problems.
For these and other reasons, there is a need for the present invention.